In standard use, nonplussed means 'surprised and confused'. In North American English , a new use has developed in recent years, meaning 'unperturbed'--more or less the opposite of its traditional meaning. This new use probably arose on the assumption that non- was the normal negative prefix and must therefore have a negative meaning. Although the use is common, it is not yet considered standard.- New Oxford American Dict.

Though interesting may be this change of meaning I am nonplussed in both its actual meanings about the fact. Words turn their cloack as easily as politicians. I've looked up the other word you smuggeled in: enantiodromia ( never heard it before and sounding very Greekish to me)

Enantiodromia. Literally, "running counter to", referring to the emergence of the unconscious opposite in the course of time. This characteristic phenomenon practically always occurs when an extreme, one-sided tendency dominates conscious life; in time an equally powerful counterposition is built up, which first inhibits the conscious performance and subsequently breaks through the conscious control. (from Wikipedia)

well, non plus is Latin for 'no more' and we originally had the noun 'nonplus', meaning a state in which no more can be said or done; then came the verb nonplus, to bring to a nonplus or standstill; to perplex, confound - et voila, much later, it is reinterpreted as not perplexed, or unperturbed.

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